Transparent glass screen fob transmitting substantially white light



D. N. MACDONALD.

TRANSPARENT GLASS SCREEN FOR TRANSIVHTTING SUBSTANTIALLY WHITE LIGHT FROM INCANDESCENT ELECTRIC LAMPS.

APPLICATION HLED JAN. l3, 1919.

1,393,804. Patentd 001;. 18, 1921.

UNiTE STilA"iilitaren't DONALD N. MACDONALD, OE BOSTON, MASSACHUSETTS.

TRANSPARENTGLASS SCREEN FOB; TRANSMITTING SUBSTANTIALLY WHITE LIGHT 7 FROM INCANDESCENT ELECTRIC LAMPS.

Patented Oct. 18, 1921.

Application filed January 13, 1919. Serial No. 270,873.

To all whom it may concern:

Be it known that I, DONALD M. Mao- DONALD, a citizen of the United States, residin at'Boston, in the county of Suli'olk and btate of Massachusetts, have invented new and useful Improvements in Transparent Glass Screens for Transmitting Substantially White Light from Incandescent Electric Lamps, of which the 'followmg 1s a specification.-

The nature of this invention is a screen adapted to be placed between a source of light and the object to be illuminated; and its object isto provide a screen of this nature which, when used in connection with incandescent electric lamps, will cause substantially white light to be cast upon the objects or things illuminated In other words, the purpose and object of my in- Vention is to enable the illumination derived from commercial incandescent electric lamps to be converted into white light, to the end that color values may be distinguished approximately as well by means of the artificial light so furnished as by the light of midday.

It is a fact well known that the light emitted from all electric lamps of the incondescent type, and by this I mean lamps in which the source of light is a glowing filament, of whatever character, and not an arc, differs to a greater or less extent from the light of the sun at midday by reason of the fact that it contains a larger proportion of red and yellow rays, that is the rays of longer wave length, than are contained in the white light of the sun. I will explain at this point thatiby the term midday I mean that portion of the day within three or four hours before or after noon, say from nine in the forenoon to three inthe afternoon; that is, the hours when the sun is highvabove the horizon, and its light is not materially modified by absorption of the at mosphere. "The difference in the quallty of the artificial light produced by such lamps from daylight is a defect whichhas serious consequences under some conditlons, particularly those where it is necessary or desirable to determine color values. For example, inthe establishments where colored fabrics and particularly silks having numerous and delicate shades of color are sold, it is impossible under the naked illumination of the lncandescent lamp either to recogmze color values in such goods or to dis tinguish from one another hues and shades of color which are readily distinguishable in daylight. The purpose, function and result of my invention is, as hereinafter more fully explained, to correct the light emitted by lamps of the character stated to the condition in which it is substantially the same as daylight atmidday, whereby its use enables color values to be accurately judged and compared under the conditions hereinbefore referred to. These conditions are the most important, necessary and difiicult, and for that reason have been referred to by way of illustration of the diiiiculties overcome and advantages secured by this inventlon.

The mode of accomplishing the objects above stated, which constitutes this invention, consists in oroviding either in combination with, or or use in connection with, a lamp or source of light of the character herein set forth, a screen which permits so much of the rays emitted from the source of light as constitutes substantially white light to pass through without obstruction, but obstructs or absorbs those rays of the longer wave length, and including red and yellow rays, which are emitted from the lamp in proportions in excess of the contents of such rays in midday light. Other characteristics of the screen, which I claim as my invention, are that the screen has the property of diffusing or scattering the light rays passing through it, whereby it does not permit an intense or concentrated illumination to pass, and that it is of clear glass and so allows substantially all of the light rays to pass except only those which are in excess of the proportional content of white light, as above set forth.

For further description of specific embodiments of the invention, reference is directed to the accompanying drawings which show such embodiments all contain ing the same principle but different in de tail.

invention applied thereto.

Fig. 4: is a cross section on a somewhat enlarged scale of a fragment of the screen illustrating the characteristics thereof. r

In Figs. 1 and 2, a and a represent screens containing the invention used in connection with incandescent electric lamps, and having the form of shades or globes which completely inclose the lamp. These views represent two of the numerous ornamental forms or designs of such globes which may be used, but, before describing them any further, I will explain the precise nature and characteristics of the material whereof such screens or globes are made. This material is glass of highly transparent quality, a fragment of which is designated 1) and is shown in cross section in Fig. 4. One surface of the glass as 0 is uneven, being formed with contiguous humps or protuberances and hollows over its entire area. These contiguous humps or projections are likewise shown at c in Figs. 1 and 2, although, for

physical the sake of clearness, they are not there shown as covering the entire surface of the screen or as being so close together and continuous as they preferably are in the actual structure. By continuous in this connection I mean that the humps merge into one another in such fashion that there is substantially no flat surface on the glass between them, whereby substantially all areas of the uneven surface are inclined to the plane of the glass. Therefore light which passes through the glass is deflected and diffused by these numerous little humps and is in effect shattered into an extreme confusion of divergent and crossing rays, whereby, although the transparency of the glass and the transmission of light through it are not thereby diminished, yet the emerging light is softly diffused in all directions and the screen gives the appearance to the eye, when the lamp back of it is illuminated, of a bright but not intense globe be ing emitted from its entire surface. By this means the source oflight is prevented from casting beams of such intensity as would make sharply defined shadows and give intense illumination with high lights upon the objects illuminated.

By the use of the term uneven in the foregoing description and the lfollowing claims, I mean simply that the surface is formed in such manner as. that it will diffuse light with more or less of the effect described. I do not by this term alone imply any limitation as to whether the humps which cause this unevenness are large or small or are regular or irregular in form or arrangement. I may make them of irregular shape, height, outline and disposition, or I may make them regular and like one another in all of these features.

. I have found in practice that the best results are obtained when the humps and hollows have an appreciable size, say an extreme width of an order of magnitude from one one-hundredth to one-eighth of an inch, the uneven surface being highly transparent as distinguished from dull or merely translucent. This quality of the glass enables it to transmit light without appreciable diminution of illuminating power, but makes it impossible for outlines to be seen through it. whereby, when used as a screen over an electric lamp, it passes the full brilliancy of the illumination in a diffused way and without casting high lights.

Another characteristic of the screen is its color; and by the term color in this connection I mean not only the color impression which the screen makes upon the eye when viewed by reflected or transmitted light, but also and more particularly the quality of absorbing or otherwise opposing passage of certain light rays. which is given to the screen of this invention appears to the eye as a very light blue. having a steely quality and possibly containing a suggestion of green. Its optical color is that which absorbs or otherwise prevents passage through it of so much of the light rays of longer wave length, the red and yellow rays, given off by the lamp as are in excess of the proportional contents of such rays in sunlight at midday. As otherwise defining the quality of this color, I would say that it is the color obtained by grinding clear cobalt blue glass to an inpalpable powder, mixing the powder so obtained with a fluid vehicle, and spreading the mixture with a brush in a thin film over the surface of the glass. I color the glass in the manner last suggested, that is by mixing cobalt blue glass powder with a liquid vehicle, for which purpose I prefer turpentine, in such relatively small quantities as will leavethe mixture still thin and highly fluid. Then I spread the color mixture upon the glass, preferably over the uneven surface thereof, and allow it to dry. After that I fix the color by fusion, heating the glass to the temperature at which it is plastic and at which the colored glass powder applied as last described will fuse into it.

The foregoing is not the only possible method by which the desired color is or may be given to the screen. It is one which I have successfully used and found satisfactory, but the color characteristic may equally well be given to the glass by distributing The color coloring matter of suitable quality in proper proportions through the molten or plastic, mass of glass in the course of manufacture before it has been rolled or blown into the sheets from which the screens are made. I will say also that it is within my contemplation to vary the color tone of the screen in accordance with the quality of the particular lamps for which the screens are made. Thus, as all incandescent lamps are not exactly alike in regard to the quality of the light generated, and some emit a larger proportion of the rays of longer wave length than others, I may provide screens of a more or less dark or intense blue color (power of absorption of rays of longer wave length) according to the quality of the particular lamp.

Screens of form, dimensions and artistic design, adapted to particular situations in connection with electric lamps, are made from screen material having the qualities and characteristics above described, in any suitable way. The screen may be fashioned into a fiat plate of glass as shown at a used in connection with an opaque reflector and screen d in the counter lamp represented in Fig. 3. The incandescent lamp or source of light is shown in conventional form at c in this figure. Or the screen may be fashioned into a bowl or casing made of a number of panels connected by grooved metal ribs 7, as shown in Fig. l; or it may be made of two bowl-like members as shown in Fig. 2, or in a single member. In fact an infinite number of designs may be made from any number of pieces of glass with or without panels, and flat or curved in any direction in a regular or irregular form. Whatever may be the form in which the screen is made, the essential characteristics previously described of the substance composing it remain, and the quality of light shed from it is the desired quality hereinbefore explained. Thus it will be understood that nothing contained in the foregoing description or illustrated in the drawings is to be implied or construed as restricting the scope of protecton hereby secured in respect to external form or other quality of design. I may observe, however, that the best effects of diffusion and even distribution of light are secured when the screen is so made that all points on its surface are nearly equidistant from the source of light, that is the inclosed incandescent lamp; and when the screen is so fashioned that the uneven surface is outward;butI do not limitmy claim to protection in any wise to these details.

lVhat I claim and desire to secure by Letters Patent is:

1. A screen or filter for correcting the color error of incandescent electric lamps consisting of glass having a light-diffusing surface and a color of substantially uniform quality throughout adapted to absorb substantially the excess of red and yellow rays emitted by such lamp over the contents of such rays in sunlight.

2. A light screen or filter for the purpose set forth, being of clear glass and having an uneven but transparent surface and of a light bluish color, the color characteristic of which is that of a dilute fused mixture of cobalt blue glass and colorless glass.

3. A light-correcting screen of material which is transparent to the rays of shorter wave length contained in white light and has the quality of partially absorbing rays of greater wave length to the extent of thus absorbing the excess content of such rays over white light contained in the light emitted from an incandescent electric lamp, and the screen having also an uneven surface comprised by contiguous humps and depressions of appreciable magnitude and in such profusion as to be continuous, adapted to difiuse and scatter light rays passing through it WllthOllt appreciably diminishing the light in transmission.

4. An illuminating lamp comprising the combination with an incandescent electric lamp of a screen formed and arranged substantially to surround said lamp and com posed of transparent glass having an irregular external light-diflusing surface, and having a bluish color characteristic adapted to absorb substantially the excess of red and yellow light rays emitted from said incandescent lamp over the contents of such rays in midday sunlight, said color characteristic being that of finely ground cobalt blue glass thinly spread over the screen and fused thereto.

5. A light-correcting and light-diffusing screen consisting ofa body of transparent colorless glass having in fused union finely divided and thinly distributed cobalt blue glass, the screen so constituted having also a multiplicity of closely adjacent surface protuberances.

In testimony whereof I have aifixed my signature.

DONALD N. MACDONALD. 

